1. Field of Invention
The present invention relates to a receiver and a receiving method in a spread spectrum communication system.
2. Description of Related Art
Recently, a spread spectrum communication system has been utilized in terrestrial mobile communication. This system transmits, on a transmitting side, a signal by multiplying a data signal by a spread code so as to spread the bandwidth of the data signal to a wider frequency bandwidth, and on a receiving side, convert the received signal to the bandwidth of the original data signal by using the same spread code as used on the transmitting side to demodulate the received signal. In this method, a psuedo noise (PN) sequence is used as the spread code. On the transmitting side, the data signal to be transmitted is narrow-band modulated, and a spread modulation is implemented by using the PN sequence, so as to transmit the signal by shifting it into a predetermined transmission bandwidth. On the receiving side, a correlation peak is detected by making a correlation between the received signal and the same PN sequence as used in the transmission so as to synchronize the received signal with a target signal.
Synchronization is an operation that matches the operation timing of a device on the receiving side with the signal sent from the transmitting side. A receiver estimates the generation timing of a spread code sequence of the received signal and matches it with the generation timing of a spread code sequence prepared within the receiver.
Conventionally, it is well known in the art to use a method of using a sliding correlator or a method of using a matched filter to accomplish synchronization (the synchronous acquisition). In the method of using the sliding correlator, the spread code sequence is generated at an appropriate timing in the receiver, and then synchronized by gradually shifting the timing thereof. However, this method requires a large time period to perform synchronization.
On the other hand, in the method of using the matched filter, a detection of a correlation value is implemented instantaneously using the matched filter so as to enable synchronization in a short time period. In this method, when receiving the spread spectrum signal on the receiving side, after the received signal has been passed through a low pass filter, such as a roll-off filter or a root nyquist filter (RNF), the signal is directly input into a digital matched filter, and the digital matched filter is operated at the chip rate.
FIG. 4 shows the configuration of a main part of a conventional spread spectrum communication receiver. In this system, it is assumed that a signal to be transmitted is spread by multiplying the signal by a spread code with a spread factor SF equal to 256, and a chip rate equal to 4 MHz.
As shown in FIG. 4, a received signal with a 6-bits width that has passed through the root nyquist filter 90 is input into a digital matched filter 92. Because the input signal has a spread factor SF equal to 256, the digital matched filter 92 requires 256 stages. The digital matched filter 92 operates for each chip rate, i.e., one chip. (One pulse or one bit of the spread code sequence is referred to as a chip.) Accordingly, the received signal to be input into the digital matched filter 92 is sampled at the chip rate, at only one point in the chip. As a result, in the case of having sampled a variable point of the chip, the correlation value will be deteriorated, and data reproduction will be impossible. As a result, the performance thereof will be deteriorated.